1. Field of the Invention
The present invention relates to an electromagnetic casting method and apparatus for continuously producing a polycrystalline silicon ingot by applying a casting technique through electromagnetic induction, particularly, an electromagnetic casting method of silicon capable of preventing a crack generation by reducing the region of precipitation of foreign substances in a finally solidified portion of the silicon ingot upon production of a polycrystalline silicon to be used as a substrate material of a solar cell, and an electromagnetic casting apparatus suitable for conducting the same.
2. Description of the Related Art
When an electromagnetic casting apparatus, to which a bottomless cold mold divided in a circumferential direction is attached, is used, since a molten substance (molten silicon in this case) and the mold are almost non-contact with each other, a silicon ingot free of impurity contamination can be continuously produced. Further, a significant reduction of production cost can be achieved owing to an advantage that no high purity material is required as a material of the mold due to less contamination from the mold and also of a continuous casting capability. Therefore, the electromagnetic casting apparatus has conventionally been applied for production of polycrystalline silicon to be used as a substrate material of a solar cell.
Moreover, in recent years, a melting method using a plasma arc heating as an auxiliary melting heat source in combination has been conducted to melt silicon raw materials charged into the mold.
FIG. 6 is a view schematically showing a configuration example of a principal part of an electromagnetic casting apparatus to be used for producing polycrystalline silicon. As shown in FIG. 6, strip-shaped elements elongated in a vertical direction, the element having an interior to be cooled with water, are arranged inside an induction heating coil 2 in a mutually-insulated state in the induction coil 2, in parallel to the winding axis direction of the induction coil 2, and a space surrounded by these strip-shaped element forms a mold (that is, a bottomless cold mold of which side wall portion is cooled with water) 1. Typically, a water cooling copper mold in which the strip-shaped element is made of copper is used as the cold mold 1.
Below the induction heating coil 2, a heat retention heater 4 for heating a solidified silicon ingot 3 to prevent an abrupt cooling is installed. Moreover, above the cold mold 1, a material supply nozzle 18 for charging silicon raw materials 17 into the mold 1 during its melting process and a plasma torch 8 configured to be vertically movable are also attached.
In order to produce a polycrystalline silicon ingot using the above-described electromagnetic casting apparatus, an alternating current is applied to the induction coil 2, and the plasma torch 8 is lowered and turned on (to be a state capable of being energized). Subsequently, with a state where a support stand (not shown) is provided at a position corresponding to the bottom portion of the cold mold 1, the silicon raw materials are charged into the mold 1 and molten by generating a plasma arc 19 between an electrode of the plasma torch 8 and the charged silicon raw materials. Since each of strip-shaped elements forming the mold 1 is electrically separated from others, a current flowing in each element forms a loop, with which a current flowing along the inner wall side of the mold 1 create a magnetic field in the mold 1, and thereby, the silicon raw materials charged into the mold can be heated and molten by plasma arc heating and induction heating. The silicon raw materials (molten silicon 6) in the mold 1 are molten without contacting the mold 1 by receiving inward force in the direction normal to the side surface of the molten silicon 6, which is generated through the interaction of a current flowing along the surface of the molten silicon 6 and a magnetic field created by a current flowing along the mold inner wall.
When the support stand is gradually moved downward after the molten silicon 6 is sufficiently homogenized, the cooling of the molten silicon 6 starts from a portion far away from an induction coil 2, and the silicon ingot 3 having the same sectional shape as that of the mold is formed. By supplying the silicon raw materials 17 from the material supply nozzle 18 in an amount corresponding to an amount of a downward movement of the support stand so as to constantly maintain the upper surface of the molten silicon 6 at the same height level and to continue heating, melting, withdrawing, and supplying the raw materials, the polycrystalline silicon ingot can be produced continuously.
It is noted that, in an electromagnetic casting method using the above-described electromagnetic casting apparatus, after the startup of casting the ingot, charging the silicon raw materials as well as induction heating and plasma arc heating of the charged materials are stopped to thereby complete the casting of the ingot, at the time that the length of the ingot reaches approximately 7 m because of constraints of facility. To be strict, such operation cannot be said as a continuous casting, however since the operation is conducted continuously from start to end of the casting, this method is also referred to as a continuous casting method (an apparatus used in this method is referred to as a continuous casting apparatus) in comparison with a conventional batch-type casting method in which molten silicon is solidified in a crucible or mold.
After completion of casting, a pull down operation of the support stand is stopped, and then a final solidification of liquid molten silicon in the mold is performed. In this operation, if the liquid molten silicon should be left in as-is condition, solidification should start from the upper surface of the molten silicon in the mold, the molten silicon confined therein should be solidified in the final phase, and thus cracking is liable to occur in the finally solidified portion of the ingot due to a volume expansion associated with the final solidification. For this reason, in the final solidification process, a carbon block is suspended as an induction incurring member above the molten silicon so that heat by induction heating can be input from above the molten silicon to allow it to be solidified from below.
FIG. 7 is a view describing a final solidification process in the production of polycrystalline silicon by electromagnetic induction. As shown in FIG. 7, immediately after charging of silicon raw materials as well as induction heating and plasma arc heating of the charged materials are stopped to complete the casting operation, an unsolidified molten silicon 6 remains above the upper portion of a solidified silicon ingot 3 (within a mold and in the vicinity thereof). A carbon block 20 is suspended above the molten silicon 6, an electric current is applied to the induction coil 2 to heat the carbon block 20 by electromagnetic induction. With this, since the molten silicon 6 is kept warm from upward, the solidification progresses from the lower portion of the molten silicon 6, in other words, upward from a solid-liquid interface 21 (indicated by an arrow in FIG. 7), and thus the upper surface of the molten silicon 6 is solidified in the final phase. It is noted that a distance h indicated by an out-lined arrow is the depth of solid-liquid interface in FIG. 7.
However, in this final solidification process, the following two problems occasionally arise.
One problem is that after the plasma arc heating is stopped, the precipitation of foreign substances is caused by an abrupt temperature drop before starting to warm the molten silicon by the carbon block.
FIG. 8 is a view conceptually showing a precipitation state of foreign substances in a final solidification process, representing a longitudinal cross section of the finally solidified portion of the silicon ingot 3, including a mold center axis. As shown in FIG. 8, the precipitation region of foreign substances extends entirely inside the solid-liquid interface 21 (that is, in the unsolidified molten silicon 6). Foreign substances mean impurities such as SiC, SiN, SiO, and C (carbon). These impurities cause a leak current failure called a shunt when a solar cell was made using a wafer cut from a silicon ingot including these impurities as a substrate.
The other problem, which may be caused in the final solidification process, is that the ingot is broken by a solidification expansion when the surface only is solidified earlier while the molten silicon is confined inside the solidified portion.
FIGS. 9A and 9B are views conceptually illustrating, a situation of cracking in an ingot caused by the confinement of molten silicon inside the ingot. As shown in FIG. 9A, in a case such that the cooling from the mold side wall is particularly intensive (indicated by arrows in FIG. 9), even if warming is performed using a carbon block, the warming effect does not reach deep into the melt pool, and the molten silicon 6 may be confined within the solidified portion. In this case, as shown in FIG. 9B, the confined molten silicon solidifies (solidified silicon 6a) and expands to cause cracking in the finally solidified portion of the ingot 3, and thus a workload to take out the ingot 3 is increased. The confinement of the molten silicon 6 occurs more likely as the depth of solid-liquid interface 21 (see FIG. 7 above) is increased at the time that the final solidification process starts.
No cases of preventive measures have been reported so far with respect to such the precipitation of foreign substances or cracking in a finally solidified portion of silicon ingot. For example, Japanese Patent Application Publication (JP-A) No. 2001-19593 discloses a continuous casting method of silicon in which electromagnetic induction heating and plasma heating with plasma arc are used in combination, so that qualities of the silicon to be used as a solar cell are improved. According to a casting method described in JP-A No. 2001-19593, a load of electromagnetic induction heating can be reduced by using a plasma heating in combination for melting raw materials in the middle of the casting process, and a solid-liquid interface is flattened by suppressing a thermal convection of molten silicon owing to an electromagnetic force so as to reduce a downward thermal flow rate. As a result, a temperature gradient in a radial direction of a silicon ingot immediately after solidification is reduced and a thermal stress generated inside a crystal is alleviated so that the generation of crystal defects, which reduces a conversion efficiency of a solar cell, is suppressed. However, the precipitation of foreign substances in the finally solidified portion of ingot, a crystalline quality degradation caused therefrom, cracking generation, and counter measures against these problems are not described in JP-A No. 2001-19593.